The present invention relates to pressure reducing valves and pressure compensated flow controls, and particularly to a load-referenced pressure reducing valve capable of adjusting hydraulic pressures as needed to meet changing load demands for the purpose of flow control at high speeds compatible with automatic control systems.
Operation of construction, agricultural and other equipment is now possible with automatic control systems, such as laser guidance and laser reference systems, which interface with existing hydraulic control systems present on the equipment. Automatic systems are capable of controlling and adjusting equipment functions faster and with greater frequency than is possible with human operators. Adjustments in hydraulically operated elements, such as wheels, arms, blades and the like, may occur, for example, 10 times per second or more. However, it has been found that conventional hydraulic control systems respond and perform less than optimally in providing such rapid control and adjustment, resulting in an over-correction or under-correction.
It has been found that one of the primary components of hydraulic systems responsible for inadequate response are flow control valve systems which control or limit flow of hydraulic fluid to operating cylinders. One known flow control valve system includes a pressure compensated spool. However, this system is problematic in automatic control situations because it functions to regulate fluid flow, but only after fluid flow has been established. Further, in the absence of flow, it typically returns to a fully open position. Thus, when a hydraulic cylinder in a system is corrected, creating a demand for the hydraulic pump to supply more pressure and flow to the cylinder, fluid surges through the open pressure compensated flow control valve system until sufficient flow allows it to begin regulating to the desired flow rate. By this time, however, the demand for oil is often over-satisfied, and the cylinder position is over-corrected. Meanwhile, the automatic control system may have signalled yet another adjustment. A series of over-corrections can result in undesirable waffling of the cylinder and poor results from unwanted equipment operation.
Examples of several other known valves which serve to reduce pressure within a system are shown in FIGS. 1-4, and will be briefly examined to highlight their operation. FIGS. 1A and 1B schematically illustrate a simple direct acting pressure reducing valve 1. This valve 1 is designed to maintain a constant outlet pressure regardless of the inlet pressure at inlet 2. The valve 1 is normally open, forced open by a spring 4 at the first end 6 of the spool 5. Spring 4 determines the valve closing pressure. A passage 8 provides outlet pressure to the second end 7 of the spool 5, and when the pressure level at the outlet 3 exceeds the spring force, the spool 5 will shift toward the closed position. As shown in FIG. 1B, this restricts the flow from inlet 2 to outlet 3. When the inlet pressure is less than or equal to the spring pressure, flow restriction and pressure drop is essentially zero, as shown in FIG. 1A. Increase in the pressure above the spring force causes the valve 1 to begin closing to restrict flow and pressure and create a pressure drop equal to the difference between inlet pressure and the spring force, as in FIG. 1B. For example, if inlet pressure is 1500 pounds per square inch (psi), and the spring pressure is 1000 psi, the spool 5 will shift to a position that restricts flow and creates a pressure drop of 500 psi, and produces an outlet pressure of 1000 psi. Where there is no flow, if the inlet pressure is less than the spring pressure, the valve 1 will stay open. If the inlet pressure is greater than the spring pressure, and there is no flow, there is a pressure drop, and pressure regulation occurs. The outlet pressure will equal the spring pressure, and close the valve 1 In valve 1, any fluid which may leak past the spool 5 and into the spring cavity 9, is drained from drain 10 to a tank (not shown) from which fluid is pumped into the system by the hydraulic pump (not shown). The drain 10 prevents a build-up of fluid and pressure which would eventually prevent the spool 5 from shifting. As may be seen in FIGS. 1A and 1B, the spring force may be changed by an adjusting screw 11.
In FIG. 2, where like numbers equal like elements, a pilot-operated valve 12, a modified valve of the type shown in FIG. 1, is shown in which a plunger 13 replaces the adjusting screw 11. The plunger 13 is used, like the adjusting screw 11, to vary the force of spring 4. As the spring 4 determines the valve closing pressure, varying its force varies the outlet pressure produced by the valve 12. The plunger arrangement includes an operator 14 to adjust the plunger 13 to produce a variable outlet pressure. Since the outlet pressure is a function of plunger displacement, it has a linear relationship which reflects the spring constant of the spring 4.
FIGS. 3A and 3B, where like numbers indicate like elements, show a pilot-operated valve 15. Outlet pressure from the outlet 3 is provided to both the second end 7 and first end 6 of valve spool 5 through a cross drill 16 and control orifice 17. Fluid from outlet 3 entering the cross drill 16 flows downward through the control orifice 17 to provide outlet pressure to the second end 7, and flows upward through the control orifice 17 to the first end 6 of the spool 5 to fill spring chamber 9. Fluid pressure in spring chamber 9 on the first end 6 of the spool 5 will prevent closing of the valve 15 until the pilot dart 18 is unseated against pilot spring 19, allowing fluid to flow past the pilot dart 18 to drain 10. Pilot spring 19 is set at a given pressure by screw 20, as shown in FIG. 3B, and for the valve 15 to begin to adjust, pressure on the second end 7 must be equal to the force of pilot spring 19. Pressure will build up on the second end 7 until the pressure difference between the first and second ends 6, 7, equals the force of spring 4, at which time the spool 5 will move to restrict fluid flow from inlet 2 and outlet 3 to produce the constant pressure drop, as also shown in FIG. 3B. For example, if the force of spring 4 is 50 psi and the force of pilot spring 19 is 950 psi, no regulation will begin occurring until the outlet pressure reaches 950 psi and the pilot dart 18 is unseated. Once pressure at end 6 of spool 5 is sufficient to unseat pilot dart 18, fluid will flow past the pilot dart 18 to drain 10. Pressure will build up on end 7 of spool 5 while remaining constant at end 6 until the difference equals the force of the spring 4. The spool 5 will shift to produce a regulated outlet pressure of 1000 psi. If there is no flow, a pressure differential is established between inlet 2 and outlet 3. If the pressure in a no-flow situation is sufficiently high to unseat the dart 18, the pressure at the first and second ends 6, 7, nonetheless tends to equilibrate, and the spool 5 regulates slightly open to supply the oil passing over the dart 18.
FIG. 4, where like numbers refer to like elements, shows a pressure compensating spool valve 21 including an orifice 22 upstream from the valve inlet 2 for flow control. In the valve 21, a spring 4 provides spring pressure at the first end 6 of the spool 5 to force the spool 5 to its open position. A sense line 23 downstream from the orifice 22 adds outlet pressure to the first end 6 of spool 5. A pilot line 24, shown schematically, provides inlet pressure upstream of the orifice 22 to the second end 7 of the spool 6. Where there is flow through the orifice 22, a pressure difference exists between the pressures in the sense line 23 and pilot line 24, the latter being greater. A minimum pressure differential must exist across the orifice 22 which is equal to or greater than the spring force, before compensation will occur. When the pressure differential across orifice 22 begins to increase above this minimum, the relative force on the second end 7 also increases, and tends to close the valve 21 against both the force of spring 4 and pressure from the sense line 23. This action reduces the flow through the orifice 22, regulating and returning the pressure differential thereacross to a fixed value. By maintaining a fixed pressure drop across orifice 22, regardless of the actual inlet and outlet pressures, a constant flow through the valve 21 is maintained. If there is little or no flow and the pressure differential is less than the needed minimum, no regulation occurs. Substantially equal pressures are provided to the first and second ends 6, 7 of spool 5, and the valve 21 remains open.
In the prior art pressure compensated flow control valve previously discussed, regulation depends on flow to establish a pressure drop, resulting in a controlled flow. Without flow, the pressure differential upon which regulation depends is not established. The time required to establish sufficient flow, begin regulation, and control flow and pressure to the needed level is too great to accurately satisfy the demands of automatic control systems. Because such valves generally remain open when there is no flow, a hydraulic system subjected to sudden demand from an operating element will tend to over-satisfy that demand before flow initiated therein is brought under control. This problem is accentuated in systems where the hydraulic pump satisfies both high and low pressures simultaneously, and the pump typically operates at the higher pressure. In such systems, when the pump is opened to a low pressure circuit in which the valves are open, an even greater surge of flow and pressure is experienced.
Accordingly, there is a need for improved hydraulic valves which provide pressure regulation independent of flow, for use in hydraulic systems which can satisfy the demands of automatic control systems which require rapid control and adjustment of operating elements.